1. Field of the Invention
The present invention relates to a position measuring system for measuring the one-dimensional, two-dimensional or three-dimensional position of an object accurately by using a phenomenon of interference of light. The invention can be applied to an input device such as a mouse, a pointer, a pen input device or a distance image input device as well as the position measuring system.
2. Description of the Related Art
Heretofore, an optical interference measurement method using interference of light has been widely used as a method for measuring a position accurately. As the optical interference measurement method, for example, there is a method in which: light emitted from a laser light source is divided into two by a beam splitter or the like; an object is irradiated with a part of the light whereas a mirror is irradiated with the other part of the light as a reference beam so that the two parts of the light are returned to the original optical path; and the reflected beam from the object and the reference beam are superposed on each other to thereby generate interference of light. This method has an advantage in that a position or displacement can be measured with resolving power of not larger than a wavelength. In this method, it is however necessary to provide optical components such as a beam splitter and a reflecting mirror. There is a problem that a large number of components are required to cause increase in cost. Furthermore, high positional accuracy is required for assembling these components. There is a problem that a great deal of labor is required to cause increase in cost. In addition, when the object is moving, an automatic tracking mechanism or the like is required for tracking the moving object. There is a disadvantage in greater increase in cost.
A laser interference measuring machine in which the number of components can be reduced has been disclosed in JP-B-4-8724.
This measuring machine uses a gradient index lens. The gradient index lens is formed by dispersion of ions into an optical glass rod so that the value of the refractive index in a position of the optical glass rod becomes lower as the position becomes farther from the center axis of the optical glass rod. For example, a semi-transparent mirror on which gold is deposited is formed at an output end on a side opposite to an input end of the gradient index lens. Interference occurs between a direct beam emitted from a laser unit and a reflected beam reflected by the semi-transparent mirror of the gradient index lens and further reflected by the output end surface of the laser unit so as to be input to the gradient index lens again. In this measuring machine, the number of components can be reduced to a certain degree compared with the aforementioned interference measurement method. It is however necessary to provide the gradient index lens and to form the semi-transparent mirror. It cannot be the that this measuring machine is satisfactory from the point of view of cost.
As another example of the optical interference measurement method, there is a method using a diffraction grating or slit. To generate interference of light by use of the diffraction grating or slit, fine processing is required because the diffraction grating or slits with a pitch equal to about a wavelength must be formed. There is a problem that these components are expensive. Further, when the diffraction grating is used, the distance from the diffraction grating to an object must be selected to be very short. If the distance is long, the diffraction phenomenon cannot be observed so that there is a problem that the diffraction grating cannot be used for position measurement. When the slit is used, the slit width is equal to about a wavelength. Accordingly, even in the case where the slit is irradiated with a laser beam, the quantity of light having passed through the slit is reduced greatly. There is a problem that it is difficult to use the slit in an ordinary environment.
On the other hand, in a position measuring method not using interference of light, it is generally difficult to perform high-sensitive high-accurate measurement. When, for example, an image of an object is picked up by a CCD and the position of the object is analyzed on a display screen, resolving power basically depends on the number of pixels in the CCD. Accordingly, the size obtained by dividing a region of a real space projected onto the CCD by the number of pixels forms basic resolving power. Roughly speaking, when an image of an object about 1.3 m×1 m is picked up by a digital camera having a CCD with 1300×1000 (1300000) pixels, basic resolving power is about 1 mm. There are however a lot of applied examples in which the resolving power of about 1 mm is not useful. For example, resolving power of 0.1 mm or smaller is required for accurately recognizing a character written with a ball point pen. Or higher resolving power is required when a device chip or the like is automatically mounted on a substrate in the case of semiconductor mounting. As other cases, there are a lot of cases where the position of an object needs to be measured accurately with resolving power of the order of submillmeter or smaller. It is however difficult to achieve submillimeter accuracy in the simple measuring method using a CCD camera or the like. If a special CCD or the like with a very large number of pixels is used or a zoom-up function is provided, the position can be measured with high accuracy. There is however a disadvantage in that the cost increases.
In three-dimensional position measurement, a measuring method using interference of light is used for measuring the position of an object with high accuracy. As described above, in the related art, there is however a problem in increase in the cost of components for forming an interference optical system, increase in positional accuracy required for assembly, increase in the number of assembling steps caused by increase in the number of components, inaptitude for measuring the position of a moving body, and so on.